(1) Field of the Invention
The present invention relates to a method of and apparatus for controlling assist air that is injected in the vicinity of an injection nozzle of a fuel injection valve, to promote atomization of a fuel injected from the injection nozzle. In particular, the invention relates to a technique of avoiding a fluctuation of engine output when supply of the assist air is started or stopped.
(2) Description of the Related Art
An apparatus for supplying assist air to an internal combustion engine is disclosed in, for example, Japanese Examined Patent Publication No. 64-9465 and Japanese Examined Utility Model Publication No. 63-18767. The conventional assist air supplying apparatus guides a portion of intake air as assist air from an intake passage on the upstream side of a throttle valve toward the vicinity of an injection nozzle of a fuel injection valve located on the downstream side of the throttle valve, to hit and atomize a fuel injected from the fuel injection valve, thereby improving combustion efficiency, reducing fuel consumption, and discharging clean exhausts.
The assist air is effective when the temperature of the engine is too low to properly atomize fuel. Since the assist air unavoidably disturbs the directivity of an injected fuel and increases a minimum intake air quantity, the assist air is unwanted and even harmful once the engine is conditioned to sufficiently atomize the injected fuel without the help of the assist air.
Accordingly, a solenoid valve, for example, is provided to start and stop the supply of assist air. The solenoid valve opens and closes an assist air passage according to engine operating conditions such as engine temperature, to supply the assist air only when needed, for example, when the engine temperature is too low.
When the assist air through the assist air passage is started or stopped, the total quantity of intake air to the engine suddenly fluctuates for the portion of the assist air, and such a sudden fluctuation in the intake air quantity suddenly changes the output torque of the engine and causes a shock.